1. Field of the Invention
The present invention relates to a planar illuminating device used as a back light for liquid crystal displays of portable office automation apparatuses such as a word processor or a personal computer.
2. Background Art
FIG. 7 illustrates an arrangement of such a conventional planar illuminating device 90. The planar illuminating device 90 comprises a rectangular flat light transmitting plate 91, two sources of light 92, 92 a flat light reflecting sheet 93 and a light diffusing sheet 94. The light transmitting plate 91 is made of a high-transparent material, such as acrylic resin, in the form of a thick plate. The sources of light 92 comprise, e.g., tubular fluorescent lamps and are arranged adjacent to the surfaces 91a, 91a of opposite side edges of the light transmitting plate 91. The light reflecting sheet 93 has the same shape as the light transmitting plate 91, is attached to and covers the entire rear surface of the light transmitting plate 91. The light reflecting sheet 93 has, e.g., a white reflecting front surface having a suitable diffusing force or diffusivity. The light diffusing sheet 94 has the same shape as the light transmitting plate 91 and a milky color and covers the entire front surface of the light transmitting plate 91 with a suitable clearance therebetween.
In the operation of the light transmitting plate 91, portions of rays of light from the sources of light 92 incident upon the edge surface 91c of the light transmitting plate 91 which has an angle of incidence less than a critical angle of incidence (approximately 45.degree.) is reflected by the front surface 91c due to a difference of the indices of refraction of the light transmitting plate 91 and the air to travel to the light reflecting sheet 93 and be reflected again by the light reflecting sheet 93 to the front surface 91c.
Since the light reflecting sheet 93 has a the suitable diffusivity, rays of light reflected from the light reflecting sheet 93 to the front surface 91c involve rays of light of the critical angle or more which pass through the front surface 91c and the light diffusing sheet 94 to illuminate the light diffusing sheet 94. Thus, the light transmitting plate 91 illuminates the entire light diffusing sheet 94 to illuminate the entire liquid crystal display (not shown) with the planar illuminating device 90 from behind the display even if part of the light diffusing sheet 94 is remote from the sources of light 92.
However, the planar illuminating device 90 involves a drawback that direct rays of light from the sources of light 92 more intensively illuminate part of the light transmitting plate 91 near the edge surfaces 91a of the light transmitting plate 91 opposite to the sources of light 92 than the other part of the light transmitting plate 91 to produce such nonuniformity in the illumination of that the illuminance of part of the light diffusing sheet 94 is as small as that part is remote from the sources of light 92.
In order to eliminate the above-described drawback, there was proposed a planar illuminating device in which the rear surface 9lb of the light transmitting plate 91 has, e.g., a large number of spot-shaped reflecting layers 93 attached thereto and the area of part of the rear surface 9lb occupied by the spot-shaped reflecting layers 93 is such that the size of a spot is directly proportional to the distance of a spot from the nearest edge surface 91a , i.e. the size of spots close to the nearest end is smaller in size than a spot further away from its nearest edge surface, as shown in FIG. 8. This arrangement involves a new drawback that the surface illuminance of the entire light diffusing sheet 94 is equalized to that of the darkest part of the light diffusing sheet 94 to reduce efficiency of the entire planar illuminating device. Therefore, measures for eliminating both the drawbacks have been desired but not achieved prior to the present invention.